1. Field of the Invention
This invention relates in general to a disk drive testing, and more particularly to a method and apparatus for testing head instability using a criterion that removes normal head fluctuation from consideration.
2. Description of Related Art
Computer manufacturers have always worked to squeeze more data into smaller spaces. That mission has produced competing and complementary data storage technology including electronic circuits, magnetic media like hard disks and tape, and optical media such as compact disks. Today, companies constantly push the limits of these technologies to improve their speed, reliability, and throughput all while reducing cost. Storage technology has come a long way, and manufacturers continue to improve its speed, reliability, and throughput. Hard disks are the most common type of mass storage device today thanks to their low cost, high speed, and relatively high storage capacity.
Most hard disk manufacturers today are concerned about system reliability. Further, computer users today have great expectations concerning the reliability of their data storage systems. Even though continual improvements in technology make data loss uncommon, it is not impossible. Yet, improvements in failure detection methods have led to steady increases in the mean time between failures (MTBF).
Of particular importance to increasing the MTBF is the focus on actively monitoring disk drive characteristics. For example, magnetic heads are commonly employed for the tasks of interacting with magnetic media for information storage and retrieval. A magnetic head includes an inductive coil disposed between a pair of magnetic poles. During the data writing mode, the current carrying coil induces magnetic flux into the magnetic poles, which in turn magnetizes a moving recording medium positioned close to the magnetic head. During the data reading mode, an MR read element utilizes the MR effect to detect magnetic flux emanating from the magnetic recording medium.
The MR head uses a thin film layer of ferromagnetic material to provide a high reading output irrespective of the relative speed between the head assembly and the magnetic recording medium. However, there is a possibility of producing a defective MR read element which can result in instability.
In a hard disk drive assembly (HDA), a general error measurement (GEM) circuit has been used in the read channel. The GEM subsystem samples raw data and stores measurement information, which may be processed or examined by the disk controller at a later time. In general, the input to the GEM subsystem is the difference between an actual sample and an estimated ideal sample. This difference may be squared and accumulated.
In prior GEM subsystems, a test failure criterion is used to detect MR head instability at the drive level. This test failure criterion is based upon the mean squared error (MSE) range. The MSE is defined as the maximum minus the minimum of the MSE readings of N times. When this range is larger than a predetermined threshold, then head is determined to be too unstable and is therefore identified as a failed device. Those skilled in the art will recognize that MR head is used generically here, and accordingly, MR head may refer to any type of MR head, e.g., anisotropic magneto-resistive (AMR) head or a giant magneto-resistive head (GMR).
The MSE range can vary even for stable heads because the MSE is always positive and its value varies over a range for different heads. Thus, normal random fluctuation of the GEM results can result in the miss-fire of the test thereby identifying a stable MR head as an unstable head. This causes a yield hit to the manufacturing of the MR heads that is higher than necessary.
It can be seen that there is a need for a method and apparatus for testing head instability using a criterion that removes normal head fluctuation from consideration.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for detecting disk drive head instability.
The present invention solves the above-described problems by providing removing normal head fluctuation from the criterion used to detect whether a head is unstable.
A method and apparatus in accordance with the principles of the present invention accumulates error signals in an accumulator, calculates a criterion having an average fluctuation removed using the accumulated error signals and compares the criterion to a predetermine threshold to determine whether the head is unstable.
Other embodiments of a method and apparatus in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the calculation of the criterion further includes determining a maximum error signal, determining a minimum error signal, determining an average error signal and computing the criterion according to:
(max+min)xe2x88x92(2xc3x97average),
wherein max is the maximum error signal, min is the minimum error signal and average is the average error signal.
Another aspect of the present invention is that the comparing further includes comparing the criterion computed according to (max+min)xe2x88x92(2xc3x97average) to the predetermine threshold and determining that the head is unstable when (max+min)xe2x88x92(2xc3x97average) is greater than the predetermined threshold.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.